/**
 * Sets a value in an existing map.
 * @param {Map}     $map   Map the value will be set in.
 * @param {String}  $key   Key the value will be set for.
 * @param {Misc}    $value Value to be set.
 */
@function map-set($map, $key, $value) {
  $new: ($key: $value);
  @return map-merge($map, $new);
}